Replying to a client over sockets (UDP) from a different process

Question

I have a server than is a \"command handler\" process. It receives messages over UDP, and delegates the work to do to a different process by communicating to that process throu

Answer 1

Your client (or a firewall in between) will likely get confused by receiving the response from a different source port than it sent the request to (as the OS will just pick a random source port for the new socket).

One way around this is to use the SO_REUSEADDR socket option in the server and explicitely bind to the correct port when sending the reply. But that would also have the undesireable side-effect that UDP requests might be redirected to one of the other processes instead of the server.

Another option (if you are using Unix/Linux) is to pass the server socket to the other processes via a unix domain socket (via sendmsg and ancillary data).

Answer 2

You should be testing for the error return, something like

if((nsfd = socket (AF_INET, SOCK_DGRAM, IPROTO_UDP)) < 0} {
    perror("at socket open");
    // I'd call exit here, probably
}
if(sendto(nsfd, buff, bread, 0, &addr, sizeof(addr) < 0){
    perror("At sendto");
}

There's a nice tutorial example here.